kbuild: refactor cmd_modversions_S

Split the code into two macros, cmd_gen_symversions_S for running
genksyms, and cmd_modversions for running $(LD) to update the object
with CRCs.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>